Corona charging device with conductive shield and insulating means on said shield



Dec. 9. 1969 R. w. BENSON ,483, 7

CORONA CHARGING DEVICE WITH CONDUCTIVE SHIELD AND INSULATING MEANS ONSAID SHIELD Filed May 11, 1966 2 Sheets-Sheet 1 FIG.

INVENTOR. ROBERT W. BENSON ATTORNEYS Dec. 9, 1969 R. w. BENSON CORONACHARGING DEVICE WITH CONDUCTIVE SHIELD AND INSULATING MEANS ON SAIDSHIELD 2 Sheets-Sheet 2 Filed May 11, 1966 INVENTOR. ROBERT W. BENSON BYW i United States Patent York Filed May 11, 1966, Ser. No. 549,312 Int.Cl. H01j 37/26 US. Cl. 25049.5 1 Claim ABSTRACT OF THE DISCLOSURE Acorona charging device having a conductive shield including side walls,a corona wire supported in insulating block at the' ends of the shieldsand a dielectric shield mounted on each edge of the conductive shield.

This invention relates to the field of xerography and, particularly, toan improved corona generating device for composing and maintaining aconstant electrostatic charge on a support surface or on a xerographicplate.

In one mode of reproduction of copy by xerographic techniques, a uniformelectrostatic charge is applied to a xerographic plate, comprising aphotoconductive insulating material on a conductive backing. Thischarged xerographic plate is then exposed to the subject matter to bereproduced, usually by conventional projection techniques. This exposureof the xerographic plate, discharges the plate areas in accordance withthe radiation intensity which reaches them and thereby creates anelectrostatic latent image on or in the plate coating which may then bedeveloped with an electroscopic materialwhich clings to the plateelectrostatically in a pattern corresponding to the latent electrostaticimage. The thus formed powder image is usually transferred to a supportsurface by superposing the support thereon and applying an electrostaticcharge to the exposed area of the support surface, whereby thexerographic powder image is electrostatically attracted and bonded tothe support surface. The support surface is then separated from thexerographic plate and the xerographic powder image on the supportsurface may then be fixed thereto by any of a variety of techniques,such as heat fusing.

In general, the electrostatic charging of the xerographic plate inpreparation for the exposure step and the electrostatic charging of thesupport surface to effect transfer are accomplished by means of coronagenerating devices whereby electrostatic charge on the order of 800-1100 volts is applied to the respective surface, in each instance. Aform of corona generating device for this purpose is disclosed inVyverberg Patent 2,836,725, issued May 27, 1958, wherein a single coronawire is connected to a high voltage source and is supported in aconductive shield that is arranged in closely spaced relation to thesurface to be charged. Suitable means are usually provided to effectrelative movement of the surface to be charged and the corona generatingdevice whereby a uniform electrostatic charge is deposited.

As is well known, the corona threshold potential and the corona currentfrom a charged wire are functions of the thickness of the wire such thatthe corona threshold increases with any increasing thickness of the wireand 3,483,372 Patented Dec. 9, 1969 the corona current for any givenpotential decreases with any increasing thickness of the wire. Inaddition, the corona threshold potential and corona current are alsoeffected directly by the deposit of dust that may accumu late on thewire or on the conductive shield and by variations of movement andionized condition of the air surrounding the wire.

In continuous automatic xerographic reproducing machines using cut sheetmaterial, such as paper, for the support surface onto which thexerographic powder image formed on the drum is transferred, a sheetstripping apparatus of the type disclosed in Rutkus et al. Patent3,062,536 is used to separate the support surface from the xerographicdrum after transfer. In this type of device, air under pressure isdirected against the leading edge of the support surface on the drum toforce it away from the drum, the support surface then peeling away fromthe drum due to its own weight. Although this high pressure air isdirected against the drum surface and the leading edge of the supportsurface in pulses of short duration, this air blast will periodicallydislodge small quantities of toner from the surface of the drum or fromthe leading edge of the support material. The loose powder willeventually settle on various components in the xerographic apparatus,and in particular, onto the corona wire and shield of the coronacharging device used at the transfer station of the xerographicapparatus? This toner powder, which can be considered dust, accumulatingon the corona wire and shield drastically effects the corona generatingcapability of the wire and causes a non-uniform and/ or non-constantelectrostatic charge to be deposited on the support surface duringtransfer.

Accordingly, the principal object of the present invention is to improvethe construction of corona generating devices whereby a uniform andconstant electrostatic charge may be deposited on a xerographic plate orother surface.

Another object of the present invention is to provide a coronagenerating device of simple and inexpensive construction that isparticularly suitable for continuous operation in automatic machines.

A further object of this invention is to improve existing coronagenerating by the addition of an easy to install insulating shield.

These and other objects of the invention are attained by employing asingle corona wire or electrode that is connected to a high voltagesource and is insulated from but supported within an angled conductiveshield of such a construction to limit the accumulation of dust on thecorona wire. The corona wire is supported on insulated blocks withinthis conductive shield in such a manner as to prevent the corona wirefrom vibrating which would cause damage to the wire. By thisarrangement, it is possible to energize the corona wire to a potentialsubstantially in excess of the corona threshold potential of the wireand, since the shield is maintained at ground potential, most of thecorona current emitted goes directly to the shield and only a smallportion thereof is effective to charge the plate or support surface. Inaddition, a dielectric shield is mounted on the angled portion of theshield to prevent a change of the electrostatic charge generated due tothe accumulation of dust.

For a better understanding of the invention as well as other objects andfeatures thereof, reference is had to the following detailed descriptionof the invention to be read in connection with the accompanying drawingswherein:

FIG. 1 illustrates schematically the transfer station of an automaticxerographic reproducing apparatus incorporating a corona generatingapparatus in accordance with the invention to apply an electrostaticcharge to a support surface advanced into contact with the xerographicplate of the apparatus;

FIG. 2 is a top view of a corona charging device constructed inaccordance with the invention;

FIG. 3 is a side elevational view of the corona charging device of FIG.2 with parts broken away; and

FIG. 4 is a cross-sectional view taken along line 44 of FIG. 2;

Referring now to FIG. 1, there is shown schematically the image transferstation of an automatic xerographic reproducing apparatus, at which, thexerographic powder image previously formed on the xerographic plate iselectrostatically transferred from the xerographic plate to a supportsurface, such as cut sheet paper.

As shown, the image transfer station includes a sheet feedingarrangement adapted to feed sheets of support material successively tothe xerographic plate in coordination with the presentation of thedeveloped image on the plate surface at the transfer station.

A Xerographic plate 1, including a photoconductive layer orlight-receiving surface on a conductive backing and formed in the shapeof a drum, is mounted to rotate in the direction indicated by the arrowto cause the drum surface sequentially to pass a plurality ofxerographic processing stations, only the transfer station being shown.

Sheets of support material 2 are advanced seriatim by feed rolls 3 and 4into contact with the xerographic drum in registration with a previouslyformed xerographic powder image on the drum.

The transfer of the xerographic powder image from the drum surface tothe sheets of support material is effected by means of a corona chargingdevice 5, constructed in accordance with the invention, that is locatedat/or immediately after the line of contact between the support materialand the rotating drum. In operation, the electrostatic field created bythe corona charging device is effective to tack the support materialelectrostatically to the drum surface, whereby the support materialmoves synchronously with the drum while in contact therewith.Simultaneously with the tacking action, electrostatic field is effectiveto attract the toner particles comprising the Xerographic powder imagefrom the drum surface and cause them to adhere electrostatically to thesurface of the support material.

Immediately subsequent to the image tranfer station, there is positioneda stripping apparatus to paper pickoff mechanism 6 for removing thesheets of support material from the drum surface. This device, which isof the type disclosed in Rutkus et al., United States Pantent 3,062,536,includes a plurality of small diameter orifices supplied withpresssurized aeriform fluid by a suitable pulsator or other device. Thepulsator is adapted to force jets of pressurized aeriform fluid throughthe outlet orifices into contact with the surface of the xerographicdrum slightly in advance of the sheet of support material to strip theleading edge of the sheet from the drum surface and to direct it onto anendless conveyor 7.

In this arrangement, toner particles can and are dislodged from eitherthe surface of the drum or from the surface of the support material.These loose toner particles still carrying an electrostatic charge areattracted to various internal elements of the apparatus, including thewire and shield of the corona generating device.

Referring now to the subject matter of the invention and, in particularto FIGS. 2, 3, and 4 illustrating a preferred embodiment of theinvention, the corona charging device includes a grounded conductingshield preferably of aluminum or stainless steel. The shield, generallydesignated 10, is of generally U-shaped cross-section and includes abottom wall 11 and side walls 12 and 13 in perpendicular relation to thebottom wall 11.

A high voltage, or corona wire 15, of any suitable non-corrosivematerial, such as stainless steel, having a uniform exterior and adiameter of appropriate size is located within the shield.

The corona wire 15 is stretched between and attached to blocks 16 and 20of suitable insulating material which are arranged between the sidewalls 12 and 13 at opposite ends thereof, and attached thereto as byscrews 24 and 25 extending through these walls into the blocks 16 and20. One end of the corona wire is secured to the screw 24 while theother end is would around or clamped against the metal pin 26 and thensecured by a screw 25 extending through the block 16.

As shown in FIGS. 2 and 3, this end of the wire is wound around orclamped against a metal pin 26 and held in position by a screw 25extending through the block 16 and is threaded into a terminal plug 28,the end of which extends into a suitable bored hole in the outboard endof block 16. The exposed portions of the terminal plug is disposed forengagement with a suitable conducting bar or source carrying the highvoltage supply.

The corona charging device is supported in spaced relation to the drumas by a bracket (not shown) suitably grounded to other elements of thexerographic apparatus, the bottom wall 11 of the shield being providedwith rails 60, which are attached to the mounting bracket within thexerographic apparatus.

Although the above-described corona charging device, will stillaccumulate dust, if operated in an area where dust is present, itsoperating characteristics, unlike previously known corona chargingdevices, will not be significantly changed by dust accumulation. This istrue because of the dielectric shields 45 and 46. iWith thisarrangement, the surf-ace may accumulate dirt and still not effect theelectrostatic charge emanated from the device. Thus the instant devicemay accumulate dust and will be insensitive to dirt accumulation. 'Eachof the side walls 12 and 13 has inwardly extending or angled portion 41and 42 respectively which affords an outlet for the corona discharge.Each of these portions 41 and 42 has a shield 45 and 46 mounted thereon.The shield is preferably formed such that it covers the interior andtheexterior surfaces of the angled portion of the conductive shields 12 and13. Each dielectric shield 45 and 46 may be formed with a thickerportion 50 at the base of each side of the shield for easy installation.

The purpose of the dielectric shield is to enable the corona dischargedevice to maintain a constant or uniform electrostatic charge that isemitted therefrom. In prior art devices when dust accumulated on theangled portion of the conductive shield, the electrostatic chargeimposed varied as the dust built up thereon. Since the conductive shieldof the instant device has a dielectric shield mounted thereon anyadditional build up or collection of dust thereon does not change theelectrostatic charge imposed by the device and therefore the instantdevice maintains a constant and uniform electrostatic charge.

While the invention has been described with reference to the structuredisclosed herein, it is not to be confined to the details set forth orthe specific environment set forth. In addition, although the structurehas been defined in terms such as bottom wall and side wall, and aparticular mounting arrangement has been shown, it is realized that thecorona charging device can be mounted in any position around theperipheral surface of the xerographic drum. Thus, for example, if thecorona charging device were mounted at the 12 oclock position instead ofthe 6 oclock position, as shown, the bottom wall of the conductiveshield could then be described as the top wall of the conductive shield.Therefore, this application is intended to cover such modifications orchanges as may 5 6 come within the purposes of the improvements or the ashield formed of a dielectric material mounted on the scope of thefollowing claims. edge of each side wall of the conductive shield saidWhat is claimed is: dielectric shield covering a portion of the insideand 1. A corona charging device for the emission of corona the outsidesurfaces of the side walls of the conducdischarge including: tiveshield.

a conductive shield comprising a bottom wall and side 5 wallsperpendicular to said bottom wall thereof, References Cited insulatingsupport blocks connected to opposite ends UNITED STATES PATENTS of saidshield between said side walls, 2836725 5/1958 Vyverberg a corona wiresecured at opposite ends to said insulating support blocks in insulatedrelation to the conductive shield and positioned within the confines ofRALPH G. NILSON, Primary Examiner the bottom and side walls, meansconnected to one end of said corona wire and SHEAR Asslstant Exammeradapted to be connected to a source of high voltage 15 U5, (:1, X R,potential, and 95-l.9; 25065.l

10 3,276,773 10/1966 Tiger et a1. 25049.5

